1. Field of the Invention
The present invention relates to an apparatus for applying a voltage to a charging member for charging a body to be charged with electrostatic charge such as a photosensitive body and a dielectric body, and to a charging apparatus provided with this voltage applying apparatus. The charging apparatus is suitable for use in an electrophotography type copying machine, an image forming apparatus such as a printer, and a process cartridge.
2. Related Background Art
In an image forming apparatus using an electrophotographic process, an image bearing member defined as a photosensitive body is selectively exposed to the light, thereby forming a latent image. The latent image is developed by a developing agent (toners), the developed image is then transferred onto a recording medium (paper and OHT, etc), and further the transferred image is fixed onto the recording medium, thus forming the image.
The toners residual on the surface of the photosensitive body with a completion of the image transfer, is scraped off by a cleaning unit, thereafter the photosensitive body surface is uniformly charged to a predetermine polarity, and at a predetermined electric potential by a charging unit, and the photosensitive body is again subjected to the photosensitive process.
A corona charger has hitherto been increasingly used as the charging unit of the image forming apparatus described above. This is used so that the corona charger is disposed in a face-to-face/non-contact relationship with the photosensitive body, of which the surface is exposed to the corona discharge generated by the corona charger, whereby the surface of the photosensitive body is charged to the predetermined polarity, and at the predetermined electric potential.
Further, in recent years, there has been utilized a contact type used for a charging apparatus in which the charging member is brought by pressure into contact with the photosensitive body because of its having such advantages that it needs lower ozone and smaller electric power than the corona charger.
The contact type charging apparatus is, as in a first conventional art example shown in FIG. 9, constructed such that the charging member involves the use of a charging roller 31, including a core bar 31a, of which a surface layer is laminated with a conductive elastic layer 31b and a high-resistance elastic layer 31c. The core bar 31a is rotatably held by a holding member 500 composed of a conductive material. The charging roller 31 is biased toward the photosensitive body by a resilient a coil spring member 501 so provided as to be contactable between the holding member 500 and an electrode 22, fixedly supported by an apparatus box body or the like, and is thus brought by a predetermined pressure into contact with the photosensitive body, and then charging roller 31 is rotationally driven with rotations of the photosensitive body. Next, a bias voltage is applied to the charging roller 31, via the coil spring 501 and the holding member 500, from the electrode 22, which is connected to the power source, thus executing a charging process.
Moreover, with a higher-speed operation and a longer life-time of the cartridge, an apparatus for effecting the charging process through a plurality of routes has recently been proposed in order to supply the charging roller 31 with the electricity more surely.
A second conventional art example as, shown in, FIG. 10, is that the coil spring 501, as in the first prior art example, biases the holding member 500 composed of the conductive material, for rotatably supporting the charging roller 31, thereby pressing the charging roller 31 against the photosensitive body. The second prior art example, however, is such a contrivance that an arm member 501a provided at one end of the coil spring 501 comes into contact with the core bar 31a of the charging roller 31. The bias voltage is applied to the coil spring 501, whereby the electricity can be supplied via two routes, i.e., a first electric supply path for applying the bias voltage to the charging roller 31 through the coil spring 501 and the holding member 500, and a second electric supply path for applying the voltage to the charging roller 31 through the arm member 501a of the coil spring 501. (U.S. Pat. No. 5,768,660).
Moreover, a third conventional art example shown in FIG. 11 has such a construction that uses a conductive holding member 500 having a shaft member 500a provided with a coil spring fitting portion 500b at its front end. A charging roller contact 504 composed of a conductive material and having contact points 504a coming into contact with a peripheral surface of a shaft 31a of the charging roller 31, and a double coil spring 505 coaxially provided with coil spring portions different in diameter. Next, the charging roller contact 504 is fitted to a shaft member 500a of the holding member 500, and the double coil spring 505 is fitted to the coil spring fitting portion 500b of the shaft member 500a. The holding member 500 is biased toward an image carrier by a first spring portion 505a of the double coil spring 505, and the charging roller contact 504 is biased toward the charging roller 31 by a second spring portion 505b of the double coil spring 505. A bias voltage is applied to the double coil spring 505, whereby the bias voltage can be applied to the charging roller 31 through two routes, i.e., a first electric supply path via the double coil spring 505, the holding member 500, and a second electric supply path via the double coil spring 505 and the charging roller contact 504. (EP-A-854,392)
Problems inherent in the conventional art described above will be explained.
For instance, in the charging apparatus, using the first electric supply path via the holding member 500 from the coil spring 501 and the second electric path in which the arm member 501a extending from one end of the coil spring 501 is brought into contact with the core bar 31a of the charging roller 31, as exemplified in the second conventional art example shown in FIG. 10, it is required that the core bar 31a of the charging roller 31 can be set comparatively long in order to make the arm member 501a abut against the core bar 31a of the charging roller 31, which results in such a problem that an entire length of the apparatus increases.
On the other hand, in the charging apparatus using the first electric supply path via the holding member 500, from the double coil spring 505 and the second electric supply path via the charging roller contact 504 from the double coil spring 505, as exemplified in the third conventional art example shown in FIG. 11, because of using the double coil spring 505, in which the spring portions are different in diameter, are coaxially provided. The problem is that the total height of the apparatus increases, corresponding to a set length of the double coil spring 505 and the second spring portion 505b and then to a wall thickness of the charging roller contact 504.
Further, the concerns about the environmental issues have increasingly grown in recent years, and hence the components of the apparatuses have been required to be reused. However, in the conventional charging apparatus, a portion of the charging roller on the side opposite to the photosensitive body with respect to a central axis of the charging roller causes frictions at its contact portion with the holding member while receiving a biasing force of the coil spring, which might therefore bring about an easy occurrence of an abrasion of the contact portion. This has been an obstacle in terms of reusing the parts.